Different molecular mechanisms for cAMP regulation of gene expression during Dictyostelium development

An integrated, cross-regulation pathway model involving activating/adaptive and feed-forward/feed-back loops for directed oscillatory cAMP signal-relay/response during the development of Dictyostelium

Self-organized and excitable signaling activities play important roles in a wide range of cellular functions in eukaryotic and prokaryotic cells. Cells require signaling networks to communicate amongst themselves, but also for response to environmental cues. Such signals involve complex spatial and temporal loops that may propagate as oscillations or waves. When Dictyostelium become starved for nutrients, cells within a localized space begin to secrete cAMP. Starved cells also become chemotactic to cAMP. cAMP signals propagate as outwardly moving waves that oscillate at ∼6 min intervals, which creates a focused territorial region for centralized cell aggregation. Proximal cells move inwardly toward the cAMP source and relay cAMP outwardly to recruit additional cells. To ensure directed inward movement and outward cAMP relay, cells go through adapted and de-adapted states for both cAMP synthesis/degradation and for directional cell movement. Although many immediate components that regulate cAMP signaling (including receptors, G proteins, an adenylyl cyclase, phosphodiesterases, and protein kinases) are known, others are only inferred. Here, using biochemical experiments coupled with gene inactivation studies, we model an integrated large, multi-component kinetic pathway involving activation, inactivation (adaptation), re-activation (re-sensitization), feed-forward, and feed-back controls to generate developmental cAMP oscillations.

Modulation of cyclic nucleotide-mediated cellular signaling and gene expression using photoactivated adenylyl cyclase as an optogenetic tool

Cyclic nucleotide signaling pathway plays a significant role in various biological processes such as cell growth, transcription, inflammation, in microbial pathogenesis, etc. Modulation of cyclic nucleotide levels by optogenetic tools has overcome certain limitations of studying transduction cascade by pharmacological agents and has allowed several ways to modulate biological processes in a spatiotemporal manner. Here, we have shown the optogenetic modulation of the cyclooxygenase 2 (Cox-2) gene expression and their downstream effector molecule (PGE₂) in HEK-293T cells and the development process of Dictyostelium discoideum via modulating the cyclic nucleotide (cAMP) signaling pathway utilizing photoactivated adenylyl cyclases (PACs) as an optogenetic tool. Light-induced activation of PACs in HEK-293T cells increases the cAMP level that leads to activation of cAMP response element-binding protein (CREB) transcription factor and further upregulates downstream Cox-2 gene expression and their downstream effector molecule prostaglandin E2. In D. discoideum, the light-regulated increase in cAMP level affects the starvation-induced developmental process. These PACs could modulate the cAMP levels in a light-dependent manner and have a potential to control gene expression and their downstream effector molecules with varying magnitude. It would enable one to utilize PAC as a tool to decipher cyclic nucleotide mediated signaling pathway regulations and their mechanism.

Chemotactic network responses to live bacteria show independence of phagocytosis from chemoreceptor sensing

Aspects of innate immunity derive from characteristics inherent to phagocytes, including chemotaxis toward and engulfment of unicellular organisms or cell debris. Ligand chemotaxis has been biochemically investigated using mammalian and model systems, but precision of chemotaxis towards ligands being actively secreted by live bacteria is not well studied, nor has there been systematic analyses of interrelationships between chemotaxis and phagocytosis. The genetic/molecular model Dictyostelium and mammalian phagocytes share mechanistic pathways for chemotaxis and phagocytosis; Dictyostelium chemotax toward bacteria and phagocytose them as food sources. We quantified Dictyostelium chemotaxis towards live gram positive and gram negative bacteria and demonstrate high sensitivity to multiple bacterially-secreted chemoattractants. Additive/competitive assays indicate that intracellular signaling-networks for multiple ligands utilize independent upstream adaptive mechanisms, but common downstream targets, thus amplifying detection at low signal propagation, but strengthening discrimination of multiple inputs. Finally, analyses of signaling-networks for chemotaxis and phagocytosis indicate that chemoattractant receptor-signaling is not essential for bacterial phagocytosis.

DOI:http://dx.doi.org/10.7554/eLife.24627.001

Cell signaling during development of Dictyostelium

Continuous communication between cells is necessary for development of any multicellular organism and depends on the recognition of secreted signals. A wide range of molecules including proteins, peptides, amino acids, nucleic acids, steroids and polylketides are used as intercellular signals in plants and animals. They are also used for communication in the social ameba Dictyostelium discoideum when the solitary cells aggregate to form multicellular structures. Many of the signals are recognized by surface receptors that are seven-transmembrane proteins coupled to trimeric G proteins, which pass the signal on to components within the cytoplasm. Dictyostelium cells have to judge when sufficient cell density has been reached to warrant transition from growth to differentiation. They have to recognize when exogenous nutrients become limiting, and then synchronously initiate development. A few hours later they signal each other with pulses of cAMP that regulate gene expression as well as direct chemotactic aggregation. They then have to recognize kinship and only continue developing when they are surrounded by close kin. Thereafter, the cells diverge into two specialized cell types, prespore and prestalk cells, that continue to signal each other in complex ways to form well proportioned fruiting bodies. In this way they can proceed through the stages of a dependent sequence in an orderly manner without cells being left out or directed down the wrong path.

Chemoattractant stimulation of TORC2 is regulated by receptor/G protein-targeted inhibitory mechanisms that function upstream and independently of an essential GEF/Ras activation pathway in Dictyostelium

Protein kinase TORC2 is regulated by Ras response to distinct stimulatory ligands. Cells insensitive to one chemoattractant for TORC2 activation remain fully responsive to other ligands. Receptor-specific inhibitory circuits in Dictyostelium are found upstream and independent of GEF/Ras and downstream, feedback, or feedforward responses.

Global stimulation of Dictyostelium with different chemoattractants elicits multiple transient signaling responses, including synthesis of cAMP and cGMP, actin polymerization, activation of kinases ERK2, TORC2, and phosphatidylinositide 3-kinase, and Ras-GTP accumulation. Mechanisms that down-regulate these responses are poorly understood. Here we examine transient activation of TORC2 in response to chemically distinct chemoattractants, cAMP and folate, and suggest that TORC2 is regulated by adaptive, desensitizing responses to stimulatory ligands that are independent of downstream, feedback, or feedforward circuits. Cells with acquired insensitivity to either folate or cAMP remain fully responsive to TORC2 activation if stimulated with the other ligand. Thus TORC2 responses to cAMP or folate are not cross-inhibitory. Using a series of signaling mutants, we show that folate and cAMP activate TORC2 through an identical GEF/Ras pathway but separate receptors and G protein couplings. Because the common GEF/Ras pathway also remains fully responsive to one chemoattractant after desensitization to the other, GEF/Ras must act downstream and independent of adaptation to persistent ligand stimulation. When initial chemoattractant concentrations are immediately diluted, cells rapidly regain full responsiveness. We suggest that ligand adaptation functions in upstream inhibitory pathways that involve chemoattractant-specific receptor/G protein complexes and regulate multiple response pathways.

Deficiency of huntingtin has pleiotropic effects in the social amoeba Dictyostelium discoideum

Huntingtin is a large HEAT repeat protein first identified in humans, where a polyglutamine tract expansion near the amino terminus causes a gain-of-function mechanism that leads to selective neuronal loss in Huntington's disease (HD). Genetic evidence in humans and knock-in mouse models suggests that this gain-of-function involves an increase or deregulation of some aspect of huntingtin's normal function(s), which remains poorly understood. As huntingtin shows evolutionary conservation, a powerful approach to discovering its normal biochemical role(s) is to study the effects caused by its deficiency in a model organism with a short life-cycle that comprises both cellular and multicellular developmental stages. To facilitate studies aimed at detailed knowledge of huntingtin's normal function(s), we generated a null mutant of hd, the HD ortholog in Dictyostelium discoideum. Dictyostelium cells lacking endogenous huntingtin were viable but during development did not exhibit the typical polarized morphology of Dictyostelium cells, streamed poorly to form aggregates by accretion rather than chemotaxis, showed disorganized F-actin staining, exhibited extreme sensitivity to hypoosmotic stress, and failed to form EDTA-resistant cell–cell contacts. Surprisingly, chemotactic streaming could be rescued in the presence of the bivalent cations Ca²⁺ or Mg²⁺ but not pulses of cAMP. Although hd⁻ cells completed development, it was delayed and proceeded asynchronously, producing small fruiting bodies with round, defective spores that germinated spontaneously within a glassy sorus. When developed as chimeras with wild-type cells, hd⁻ cells failed to populate the pre-spore region of the slug. In Dictyostelium, huntingtin deficiency is compatible with survival of the organism but renders cells sensitive to low osmolarity, which produces pleiotropic cell autonomous defects that affect cAMP signaling and as a consequence development. Thus, Dictyostelium provides a novel haploid organism model for genetic, cell biological, and biochemical studies to delineate the functions of the HD protein.

Genetic evidence in humans and mouse models of Huntington's disease suggests that the disease mutation confers a deleterious gain-of-function on huntingtin that acts through the deregulation of some aspect of the protein's normal function(s). While huntingtin's function is poorly understood, its evolutionary conservation makes investigation of its physiological role in lower organisms an attractive route that has yet to be fully exploited. Therefore, we have used Dictyostelium discoideum to study the consequences of huntingtin (hd) deficiency. Developing Dictyostelium cells chemotax to form a multicellular slug that forms a fruiting body, comprising dormant spores encased above dead stalk cells. We found that hd⁻ cells were hypersensitive to hypoosmotic stress. When starved, hd⁻ cells aggregate by accretion, showed disorganized F-actin, and failed to form EDTA-resistant cell–cell contacts. Surprisingly, chemotactic signaling was rescued with Ca²⁺ or Mg²⁺ but not pulses of cAMP. Development of hd⁻ mutants produced small fruiting bodies with round, defective spores, and when mixed with wild-type cells they didn't differentiate into spores. Our results are consistent with mammalian studies that show huntingtin is a multifunctional protein involved in many biochemical processes; and, importantly, they establish Dictyostelium as a valuable experimental organism for exploring in biochemical detail huntingtin's normal function(s).

Biochemical responses to chemoattractants in Dictyostelium: ligand-receptor interactions and downstream kinase activation

Dictyostelium discoideum is one of the most facile eukaryotic systems for the study of chemotactic response to secreted chemical ligands. Dictyostelium grow as individual cells, using bacteria and fungi as primary nutrient sources; during growth, Dictyostelium moves directionally toward folate, a bacterial byproduct. Upon nutrient depletion Dictyostelium initiates a multicellular development program characterized by the production and secretion of cAMP. Cell surface receptors specifically recognize extracellular cAMP, which serves as both a morphogen to promote development and a chemoattractant to organize multicellularity. We discuss several approaches for the study of ligand-receptor interaction, with focus on affinity class determination and quantification of ligand binding sites (i.e., receptors) per cell. We further present examples for the application of biochemical assays to characterize the ligand-induced kinase activation of PI3K, GSK3, and ERK2.

Constitutively active G protein-coupled receptor mutants block dictyostelium development

cAR1, a G protein-coupled receptor (GPCR) for cAMP, is required for the multicellular development of Dictyostelium. The activation of multiple pathways by cAR1 is transient because of poorly defined adaptation mechanisms. To investigate this, we used a genetic screen for impaired development to isolate four dominant-negative cAR1 mutants, designated DN1-4. The mutant receptors inhibit multiple cAR1-mediated responses known to undergo adaptation. Reduced in vitro adenylyl cyclase activation by GTPgammaS suggests that they cause constitutive adaptation of this and perhaps other pathways. In addition, the DN mutants are constitutively phosphorylated, which normally requires cAMP binding and possess cAMP affinities that are approximately 100-fold higher than that of wild-type cAR1. Two independent activating mutations, L100H and I104N, were identified. These residues occupy adjacent positions near the cytoplasmic end of the receptor's third transmembrane helix and correspond to the (E/D)RY motif of numerous mammalian GPCRs, which is believed to regulate their activation. Taken together, these findings suggest that the DN mutants are constitutively activated and block development by turning on natural adaptation mechanisms.

Galpha-mediated inhibition of developmental signal response

BACKGROUND

Seven-transmembrane receptor (7-TMR)-G protein networks are molecular sensors of extracellular signals in all eukarya. These pathways cycle through activated (sensitized) and inhibited (desensitized) states, and, while many of the molecular components for signal activation have been described, inhibitory mechanisms are not well characterized. In Dictyostelium, 7-TM cAMP receptors direct chemotaxis and development but also regulate the periodic synthesis of their own ligand, the chemoattractant/morphogen cAMP. We now demonstrate through loss-of-function/gain-of-function studies that the novel heterotrimeric Galpha9 protein subunit regulates an inhibitory pathway during early Dictyostelium development for the cAMP signal response.

RESULTS

galpha9 null cells form more cAMP signaling centers, are more resistant to compounds that inhibit cAMP signaling, and complete aggregation sooner and at lower cell densities than wild-type cells. These phentoypes are consistent with the loss of an inhibitory signaling pathway during development of galpha9 null cells. Cells expressing constitutively activated Galpha9 are defective in cAMP signaling center formation and development at low cell density and display an increased sensitivity to cAMP signal inhibition that is characteristic of enhanced suppression of the cAMP signal response. Finally, we demonstrate that galpha9 null cells, which have been codeveloped with a majority of wild-type cells, primarily establish cAMP signaling centers and are able to non-autonomously direct wild-type cells to adopt a galpha9 null-like phenotype.

CONCLUSIONS

We suggest that Galpha9 functions in an inhibitory-feedback pathway that regulates cAMP signaling center formation and propagation. Galpha9 may be part of the mechanism that regulates lateral signal inhibition or that modulates receptor desensitization.

CRTF is a novel transcription factor that regulates multiple stages of Dictyostelium development

During aggregation, Dictyostelium establish nanomolar oscillation waves of extracellular cAMP, but as development progresses, cells become responsive to higher, non-fluctuating concentrations of cAMP. The regulation of the promoter responsible for expression of cAMP receptor subtype 1, CAR1, during aggregation reflects these signaling variations. Transcription of CAR1 from the early, aggregation promoter is activated by cAMP pulsing, but is repressed by continuous exposure to micromolar concentrations of cAMP. Deletion and mutation analyses of this promoter had defined an element essential for cAMP-regulated expression, and mobility shift assay, DNA crosslinking and DNase I footprinting experiments had identified a nuclear protein (CRTF) with zinc-dependent sequence binding specificity. In our study, CRTF was purified to homogeneity, peptides were sequenced and full-length cDNAs were obtained. The deduced CRTF protein is ∼100 kDa with a C-terminal, zinc finger-like motif required for DNA binding; CRTF purified from cells, however, represents only a 40 kDa C-terminal fragment that retains DNA-binding activity.

As might have been predicted if CRTF were essential for the regulation of CAR1, crtf-null strains fail to develop under standard conditions or to exhibit induced expression of CAR1 or other cAMP-regulated genes. Furthermore, crtf-nulls also fail to sporulate, even under conditions that bypass the dependence on early cAMP signaling pathways. In addition, early developmental events of crtf-null strains could be rescued with exogenous cAMP treatment, constitutive expression of CAR1 or co-development with wild-type cells; however, these treatments were insufficient to promote sporulation. This suggests a cell-autonomous role for CRTF during late development that is separate from its capacity to control CAR1 expression. Finally, ablation of CRTF promotes a precocious induction of certain cAMP-dependent gene expression pathways. We suggest that CRTF may function to help insulate distinct pathways from simultaneous and universal activation by cAMP. CRTF, thus, exhibits multiple complex and independent regulatory functions during Dictyostelium development.

Non-autonomous regulation of a graded, PKA-mediated transcriptional activation signal for cell patterning

The pseudoplasmodium or migrating slug of Dictyostelium is composed of non-terminally differentiated cells, organized along an anteroposterior axis. Cells in the anterior region of the slug define the prestalk compartment, whereas most of the posterior zone consists of prespore cells. We now present evidence that the cAMP-dependent protein kinase (PKA) and the RING domain/leucine zipper protein rZIP interact genetically to mediate a transcriptional activation gradient that regulates the differentiation of prespore cells within the posterior compartment of the slug. PKA is absolutely required for prespore differentiation. In contrast, rZIP negatively regulates prespore patterning; rzpA- cells, which lack rZIP, have reduced prestalk differentiation and a corresponding increase in prespore-specific gene expression. Using cell-specific markers and chimaeras of wild-type and rzpA- cells, we show that rZIP functions non-autonomously to establish a graded, prespore gene activation signal but autonomously to localize prespore expression. Overexpression of either the catalytic subunit or a dominant-negative regulatory subunit of PKA further demonstrates that PKA lies within the intracellular pathway that mediates the extracellular signal and regulates prespore patterning. Finally, we show that a 5′-distal segment within a prespore promoter that is responsive to a graded signal is also sensitive to PKA and rZIP, indicating that it acts directly at the level of prespore-specific gene transcription for regulation.

Sequence-specific protein interaction with a transcriptional enhancer involved in the autoregulated expression of cAMP receptor 1 in Dictyostelium

Major stages of Dictyostelium development are regulated by secreted, extracellular cAMP through activation of a serpentine receptor family. During early development, oscillations of extracellular cAMP mobilize cells for aggregation; later, continuous exposure to higher extracellular cAMP concentrations downregulates early gene expression and promotes cytodifferentiation and cell-specific gene expression. The cAMP receptor 1 gene CAR1 has two promoters that are differentially responsive to these extracellular cAMP stimuli. The early CAR1 promoter is induced by nM pulses of cAMP, which in turn are generated by CAR1-dependent activation of adenylyl cyclase (AC). Higher, non-fluctuating concentrations of cAMP will adapt this AC stimulus-response, repress the activated early promoter and induce the dormant late promoter. We now identify a critical element of the pulse-induced CAR1 promoter and a nuclear factor with sequence-specific interaction. Mutation of four nucleotides within the element prevents both in vitro protein binding and in vivo expression of an otherwise fully active early CAR1 promoter and multimerization of the wild-type, but not mutant, sequence will confer cAMP regulation to a quiescent heterologous promoter. These cis and trans elements, thus, constitute a part of the molecular response to the cAMP transmembrane signal cascade that regulates early development of Dictyostelium.

Control of cell-type specific gene expression in Dictyostelium by the general transcription factor GBF

To understand how positional information within an organism specifies patterning during development, we are analyzing spatially regulated gene expression in Dictyostelium. CAR3 is a member of the cAMP, 7-span receptor family which directs the transition from unicellular to multicellular organism and regulates cellular differentiation and pattern formation. CAR3 mRNA is expressed maximally at 8–10 hours of development, as individual cells aggregate and differentiate, and is accumulated to equivalent levels in all cells. CAR3 is also induced in shaking cultures by response to extracellular cAMP. We now show, by extensive mutagenesis, that the maximum length of contiguous sequences required for accurate spatiotemporal regulation of CAR3 is approx. 350 bp. These sequences include three significant elements located in upstream and transcribed regions. Arrays of G-boxes (GBF regulatory sites) are centered near positions −165 and +50 and, although either is sufficient for induction by cAMP and expression in prespore cells, both are required for expression in prestalk cells. Another GC-rich element near position −80 is required for maximal expression of prespore-specific constructs, although full-length promoters carrying clustered mutations through the −80 region are still expressed in all cells, but with slightly reduced expression. Spatiotemporal expression of CAR3 during development, thus, requires cell-specific combinatorial interactions of multiple but redundant regulatory components. These essential elements are located in upstream and transcribed regions. However, most surprisingly, a primary control for spatial patterning of CAR3 expression appears to be mediated by GBF, a general transcription factor expressed ubiquitously during Dictyostelium development following early aggregation.

G alpha 3 regulates the cAMP signaling system in Dictyostelium

The Dictyostelium discoideum developmental program is initiated by starvation and its progress depends on G-protein-regulated transmembrane signaling. Disruption of the Dictyostelium G-protein alpha-subunit G alpha 3 (g alpha 3-) blocks development unless the mutant is starved in the presence of artificial cAMP pulses. The function of G alpha 3 was investigated by examining the expression of several components of the cAMP transmembrane signaling system in the g alpha 3- mutant. cAMP receptor 1 protein, cyclic nucleotide phosphodiesterase, phosphodiesterase inhibitor, and aggregation-stage adenylyl cyclase mRNA expression were absent or greatly reduced when cells were starved without exogenously applied pulses of cAMP. However, cAMP receptor 1 protein and aggregation-stage adenylyl cyclase mRNA expression were restored by starving the g alpha 3- cells in the presence of exogenous cAMP pulses. Adenylyl cyclase activity was also reduced in g alpha 3- cells starved without exogenous cAMP pulses compared with similarly treated wild-type cells but was elevated to a level twofold greater than wild-type cells in g alpha 3- cells starved in the presence of exogenous cAMP pulses. These results suggest that G alpha 3 is essential in early development because it controls the expression of components of the transmembrane signaling system.

Autonomous and nonautonomous regulation of axis formation by antagonistic signaling via 7-span cAMP receptors and GSK3 in Dictyostelium

Early during Dictyostelium development a fundamental cell-fate decision establishes the anteroposterior (prestalk/prespore) axis. Signaling via the 7-transmembrane cAMP receptor CAR4 is essential for creating and maintaining a normal pattern; car4-null alleles have decreased levels of prestalk-specific mRNAs but enhanced expression of prespore genes. car4- cells produce all of the signals required for prestalk differentiation but lack an extracellular factor necessary for prespore differentiation of wild-type cells. This secreted factor decreases the sensitivity of prespore cells to inhibition by the prestalk morphogen DIF-1. At the cell autonomous level, CAR4 is linked to intracellular circuits that activate prestalk but inhibit prespore differentiation. The autonomous action of CAR4 is antagonistic to the positive intracellular signals mediated by another cAMP receptor, CAR1 and/or CAR3. Additional data indicate that these CAR-mediated pathways converge at the serine/threonine protein kinase GSK3, suggesting that the anterior (prestalk)/posterior (prespore) axis of Dictyostelium is regulated by an ancient mechanism that is shared by the Wnt/Fz circuits for dorsoventral patterning during early Xenopus development and establishing Drosophila segment polarity.

rZIP, a RING-leucine zipper protein that regulates cell fate determination during Dictyostelium development

rZIP is an approx. 32 kDa, multi-domain protein of Dictyostelium discoideum whose structural motifs include a RING (zinc-binding) domain, a leucine zipper, a glutamine repeat, an SH3-binding region and a consensus phosphorylation site for MAP kinase. In vitro, rZIP forms homodimers and interacts specifically with the SH3 domain(s) of the Nck adaptor protein. rZIP is expressed maximally during cell differentiation at approximately equivalent levels in all cells. Disruption of the rZIP gene rzpA results in altered cellular aggregation, impaired slug migration, and aberrant patterning of prespore and prestalk cells, the major progenitor classes. In rzpA- strains, prespore-specific genes are overexpressed and prestalk expression zones are reduced. Conversely, constitutive overexpression of rzpA markedly decreases prespore-specific gene expression and significantly increases the expression of prestalk-specific genes. Further, induced transdifferentiation of prespore cells into prestalk cells is inhibited in rzpA-slugs. In light of these patterning defects, we suggest that the RING/zipper protein rZIP plays an important role in early cell fate decisions in Dictyostelium, acting as a positive regulator of prestalk differentiation and an inhibitor of prespore differentiation.

Interacting signaling pathways controlling multicellular development in Dictyostelium

cAMP functions as the key extracellular signaling molecule controlling Dictyostelium development acting through classic G-protein-coupled/serpentine receptors. Whereas aggregation is controlled by nanomolar pulses of cAMP, a more continuous micromolar signal controls multicellular differentiation by activating a transcriptional cascade via a receptor-mediated but non G-protein-coupled pathway. Potential mechanisms by which extracellular cAMP functions to differentially control aggregation followed by morphogenesis and cell-type differentiation are discussed. This review also summarizes new findings elucidating pathways controlling cell-type regulation in this organism, including signaling cascades mediated by glycogen synthase kinase 3 and cAMP-dependent protein kinase, key regulators of cell-type differentiation in metazoans, and newly identified transcription factors.

Calcium uptake and gp80 messenger RNA destabilization follows cAMP receptor down regulation in Dictyostelium discoideum

The mechanism by which high concentrations of cAMP selectively destabilize the gp80 mRNA in Dictyostelium discoideum was investigated. This treatment which leads to down-regulation of the cAMP receptor was also found to cause an increase in calcium uptake. Given this observation, we sought a role for calcium as a second messenger in the degradation of the gp80 mRNA. Changes in the mRNA levels were examined after treating cells with compounds known to alter their intracellular Ca2+ concentrations. This included the use of A23187, Ca2+, 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate HC1 (TMB-8), LiCl and 8-p-chlorophenylthioadenosine 3',5'-cyclic monophosphate (ClPhS-Ado-3':5'-P). The sum of the data suggest that it is the cAMP-induced influx of Ca2+ across the plasma membrane, as apposed to a cAMP-mediated release of Ca2+ from intracellular stores, that initiates gp80 mRNA degradation. Treatment of cells with Concanavalin A (ConA) to induce cAMP receptor down-regulation, also causes a reduction in gp80 mRNA levels and an increase in calcium uptake.

Intracellular adenosine 3',5'-phosphate formation is essential for down-regulation of surface adenosine 3',5'-phosphate receptors in Dictyostelium

Dictyostelium discoideum cells contain cell surface cyclic AMP (cAMP) receptors that bind cAMP as a first messenger and intracellular cAMP receptors that bind cAMP as a second messenger. Prolonged incubation of Dictyostelium cells with cAMP induces a sequential process of phosphorylation, sequestration and down-regulation of the surface receptors. The role of intracellular cAMP in down-regulation of surface receptors was investigated. Down-regulation of receptors does not occur under conditions that specifically inhibit the formation of intracellular cAMP (the drug caffeine or mutant cells lacking adenylate cyclase) or conditions that inhibit the function of intracellular cAMP (mutants lacking protein kinase A activity). Cell-permeable non-hydrolysable cAMP derivatives were used to investigate further the requirement of intracellular cAMP for down-regulation. The Sp isomer of 6-thioethylpurineriboside 3',5'-phosphorothioate (6SEth-cPuMPS) does not bind to the surface receptor, enters the cell and has relative high affinity for protein kinase A. 6SEth-cPuMPS alone has no effect on down-regulation. However, together with an agonist of the surface receptor, the analogue induces down-regulation in caffeine-treated wild-type cells and in mutant cells lacking adenylate cyclase, but not in mutant cells lacking protein kinase A. These results indicate that intracellular cAMP formation and activation of protein kinase A are essential for down-regulation of the surface cAMP receptor.

Progression of an inductive signal activates sporulation in Dictyostelium discoideum

spiA, a marker for sporulation, is expressed during the culmination stage of Dictyostelium development, when the mass of prespore cells has moved partly up the newly formed stalk. Strains containing a full-length spiA promoter/lacZ fusion were stained for beta-galactosidase activity at intervals during development. The results indicate that expression of spiA initiates in prespore cells at the prestalk/prespore boundary (near the apex) and extends downward into the prespore mass as culmination continues. A spatial gradient of staining expands from the top of the prespore mass and intensifies until the front of activation reaches the bottom, whereupon the entire region stains darkly. The spiA promoter can be deleted to within 301 bp of the transcriptional start site with no effect on the relative strength, timing or spatial localization of expression. Further 5′ deletions from −301 to −175 reduce promoter strength incrementally, although timing and spatial expression are not affected. Deletions to −159 and beyond result in inactive promoters. Treatment of early developmental structures with 8-Br-cAMP in situ activates the intracellular cAMP-dependent protein kinase (PKA) and precociously induces spiA expression and sporulation. The absence of an apparent gradient of staining in these structures suggest that PKA is equivalently activatable throughout the prespore region and that all prespore cells are competent to express spiA. Thus, we postulate that the pattern of expression of spiA reveals the progression of an inductive signal for sporulation and suggest that this signal may originate from the prestalk cells at the apex.

Analysis of a novel cyclic Amp inducible prespore gene in Dictyostelium discoideum: evidence for different patterns of cAMP regulation

The D7 cDNA clone hybridizes to a 2.8 kb mRNA which first appears at the mound stage of development in the cellular slime mold Dictyostelium discoideum. This gene which is cyclic AMP (cAMP) inducible and is expressed specifically in the prespore cells contains an open reading frame interrupted by only one intron. The predicted amino acid sequence indicates a novel prespore protein which differs from all of the previously described prespore proteins in that it contains no internal repeats and does not share any homology with any of the other prespore genes. The amino acid sequence predicts a protein of 850 amino acids with a molecular weight of 95,343 daltons and an isoelectric point of 4.25. The protein is very rich in glutamine (13.8%), asparagine (10.6%) and glutamic acid (10.4%) with one potential glycosylation site and 28 possible sites for phosphorylation. The amino terminus is hydrophobic with characteristics of a signal sequence while the entire carboxyl half of the protein is notable for its hydrophilicity. Comparison of cAMP regulation of the D7 gene with the regulation of two other cAMP regulated prespore genes, the PL3(SP87) gene and the Psa(D19), reveals some striking differences. Disaggregation in the presence of cAMP results in transient degradation of mRNA for all three genes. The transcription rate for the D7 and PsA(D19) genes remains relatively unaffected by disaggregation but there is a rapid although transient decline in the transcription rate of the PL3(SP87) gene. Although the accumulation of all three mRNAs is first detectable at mound stage, transcription of the D7 and PsA(D19) genes is detected earlier in development, at rippling aggregate stage several hours prior to the earliest time when transcription of the PL3(SP87) gene is detected. Analysis of the promoter region of the D7 gene reveals three CA like boxes flanked by direct repeats as well as four G rich regions that may serve as regulatory elements.

The cAMP receptor CAR4 regulates axial patterning and cellular differentiation during late development of Dictyostelium

Pseudoplasmodia of developing Dictyostelium are organized with anteroposterior polarity. We have isolated CAR4, the gene for a new cell-surface, G protein-linked cAMP receptor. CAR4 mRNA is initially expressed during tip elongation and continues to accumulate into culmination. CAR4 is maximally expressed in pseudoplasmodia anteriors which are centers for extracellular cAMP signaling and for organization of cellular patterning. Although car4 null cells progress unperturbed through early development, they exhibit major patterning aberrations as the anteroposterior axis becomes established. Prestalk gene expression is significantly reduced in car4 nulls, whereas prespore-specific markers are overexpressed and detected in zones normally restricted to prestalk cells. Patterning defects are similarly apparent in terminally differentiated fruiting bodies. Our results show that cAMP signaling is required for pattern formation and cellular differentiation during late Dictyostelium development.

Developmental decisions in Dictyostelium discoideum

A few hours after the onset of starvation, amoebae of Dictyostelium discoideum start to form multicellular aggregates by chemotaxis to centers that emit periodic cyclic AMP signals. There are two major developmental decisions: first, the aggregates either construct fruiting bodies directly, in a process known as culmination, or they migrate for a period as "slugs." Second, the amoebae differentiate into either prestalk or prespore cells. These are at first randomly distributed within aggregates and then sort out from each other to form polarized structures with the prestalk cells at the apex, before eventually maturing into the stalk cells and spores of fruiting bodies. Developmental gene expression seems to be driven primarily by cyclic AMP signaling between cells, and this review summarizes what is known of the cyclic AMP-based signaling mechanism and of the signal transduction pathways leading from cell surface cyclic AMP receptors to gene expression. Current understanding of the factors controlling the two major developmental choices is emphasized. The weak base ammonia appears to play a key role in preventing culmination by inhibiting activation of cyclic AMP-dependent protein kinase, whereas the prestalk cell-inducing factor DIF-1 is central to the choice of cell differentiation pathway. The mode of action of DIF-1 and of ammonia in the developmental choices is discussed.

Two cAMP receptors activate common signaling pathways in Dictyostelium

Multiple signal transduction pathways within a single cell may share common components. In particular, seven different transmembrane helix receptors may activate identical pathways by interacting with the same G-proteins. Dictyostelium cells respond to cAMP using one such receptor, cAR1, coupled by a typical heterotrimeric G-protein to intracellular effectors. However, cells in which the gene for cAR1 has been deleted are unexpectedly still able to respond to cAMP. This implies either that certain responses are mediated by a different receptor than cAR1, or alternatively that a second, partially redundant receptor shares some of the functions of cAR1. We have examined the dose response and ligand specificity of one response, cAMP relay, and the dose response of another, cyclic GMP synthesis. In each case, the EC50 was approximately 100-fold higher and the maximal response was smaller in car1- than wild-type cells. These data indicate that cAR1 normally mediates responses to cAMP. The ligand specificity suggests that the responses seen in car1- mutants are mediated by a second receptor, cAR3. To test this hypothesis, we constructed a cell line containing deletions of both cAR1 and cAR3 genes. As predicted, these lines are totally insensitive to cAMP. We conclude that the functions of the cAR1 and cAR3 receptors are partially redundant and that both interact with the same heterotrimeric G-protein to mediate these and other responses.

Isolation of cDNAs for perilipins A and B: sequence and expression of lipid droplet-associated proteins of adipocytes

The major cAMP-dependent protein kinase (A-kinase) substrate in adipocytes is perilipin, a protein found exclusively at the surface of the lipid storage droplets. Using anti-perilipin serum, we have isolated two related classes of full-length coding cDNAs, designated perilipin A and B, from a rat adipocyte cDNA expression library. The two cDNAs derive from two mRNA species that arise by differential splicing. The mRNAs are predicted to encode perilipins A and B, proteins of 517 aa (56,870 Da) and 422 aa (46,420 Da), respectively, which share a common 406-aa N-terminal sequence. The predicted perilipin A contains peptides present in proteolytic digests of the purified 62-kDa form of perilipin from rat adipocytes, as well as the requisite consensus A-kinase phosphorylation sites. Like perilipin A, the B form is expressed in adipocytes and is associated with lipid storage droplets. Modeling of predicted secondary structures fails to reveal an underlying basis for the tenacious association of perilipins with lipid droplets. These proteins exhibit a significant sequence relationship (approximately 65% similarity through 105 aa) with only one other known protein, the adipocyte differentiation-related protein (ADRP). Like the perilipins, ADRP appears to be adipocyte-specific, which suggests that they interact in a related intracellular pathway. The molecular probes for perilipins A and B described here will permit detailed analyses of their functional role(s) in lipid metabolism.

Two transmembrane signaling mechanisms control expression of the cAMP receptor gene CAR1 during Dictyostelium development

Dictyostelium discoideum is among the best characterized organisms for the study of receptor/guanine nucleotide binding protein-mediated control of differentiation. Dictyostelium grow unicellularly but form fully differentiated multicellular organisms through a developmental program regulated by secreted cAMP activating specific cell-surface receptors. Dictyostelium respond differentially to cAMP at different developmental stages. During early development, expression of certain genes is induced by low-level oscillations of extracellular cAMP. Later, continuous, high cAMP concentrations will promote expression of specific genes in multicellular structures. Here, we show that the cAMP receptor gene CAR1, which is essential for development, utilizes two promoters that are activated at distinct stages of development and respond to different extracellular cAMP conditions. One promoter is active with low-level oscillations of cAMP; exposure to high cAMP concentrations will repress this promoter and induce a second promoter. The CAR1 mRNAs are alternatively spliced but encode identical proteins. Thus, through differential sensitivity to its own ligand, cAMP, two promoters and alternative splicing regulate CAR1 expression during Dictyostelium development.

CAR2, a prestalk cAMP receptor required for normal tip formation and late development of Dictyostelium discoideum

Extracellular cAMP serves as a primary signaling molecule to regulate the development of Dictyostelium discoideum. It is required for chemotaxis, aggregation, cytodifferentiation, and morphogenetic movement. The receptors for cAMP are members of the family of cell-surface receptors that are linked to G proteins and characterized by seven putative transmembrane domains. Previously, we have isolated the gene for the cAMP receptor subtype 1 (CAR1) from Dictyostelium and suggested that several genes related to CAR1 were present in the genome. Here, we describe a family of cAMP receptor genes of Dictyostelium and the isolation and function of the gene for the cAMP receptor subtype 2, CAR2. CAR2 is structurally similar to CAR1. Overall, their transmembrane and loop domains are approximately 75% identical in amino acid sequence; however, their carboxyl termini are quite dissimilar; CAR2 possesses homopolymeric runs of histidines and asparagines that are absent from the corresponding region in CAR1. Although CAR1 is maximally expressed during the early stages of development, CAR2 is expressed only after cells have aggregated and, then, preferentially in prestalk cells. Transgenic Dictyostelium that have had their wild-type CAR2 gene replaced by a defective copy using homologous recombination proceed through early development but are detained at the tight mound stage. CAR2 may be required for cAMP-directed sorting of prestalk cells during pattern formation within the aggregation mound. Furthermore, although prestalk genes are expressed normally in aggregates that lack CAR2, they exhibit an enhanced expression of prespore-specific mRNA. Previously, we had shown that there was a requirement for CAR1 during early development. The present results demonstrate that the multiple responses of Dictyostelium to cAMP are regulated by distinct cAMP receptors that are encoded by unique genes.

Regulation of the Dictyostelium glycogen phosphorylase 2 gene by cyclic AMP

A crucial developmental event in the cellular slime mold, Dictyostelium discoideum, is glycogen degradation. The enzyme that catalyzes this degradation, glycogen phosphorylase 2 (gp-2), is developmentally regulated and cAMP appears to be involved in this regulation. We have examined several aspects of the cAMP regulation of gp-2. We show that addition of exogenous cAMP to aggregation competent amoebae induced the appearance of gp-2 mRNA. The induction of gp-2 mRNA occurred within 1 and 1.5 h after the initial exposure to cAMP. Exposure to exogenous cAMP concentrations as low as 1.0 microM could induce gp-2 mRNA. We also examined the molecular mechanism through which cAMP induction of gp-2 occurs. Induction of gp-2 appears to result from a mechanism that does not require intracellular cAMP signaling, and may occur directly through a cAMP binding protein without the requirement of any intracellular signalling. We also examined the promoter region of the gp-2 gene for cis-acting elements that are involved in the cAMP regulation of gp-2. A series of deletions of the promoter were fused to a luciferase reporter gene and then analyzed for cAMP responsiveness. The results indicated that a region from -258 nucleotides to the transcriptional start site is sufficient for essentially full activity and appears to carry all necessary cis-acting sites for cAMP induction. Further deletion of 58 nucleotides from the 5' end, results in fivefold less activity in the presence of cAMP. Deletion of the next 104 nucleotides eliminates the cAMP response entirely.

Amino acid substitutions in the Dictyostelium G alpha subunit G alpha 2 produce dominant negative phenotypes and inhibit the activation of adenylyl cyclase, guanylyl cyclase, and phospholipase C

Previous studies have demonstrated that the Dictyostelium G alpha subunit G alpha 2 is essential for the cAMP-activation of adenylyl cyclase and guanylyl cyclase and that g alpha 2 null mutants do not aggregate. In this manuscript, we extend the analysis of the function of G alpha 2 in regulating downstream effectors by examining the in vivo developmental and physiological phenotypes of both wild-type and g alpha 2 null cells carrying a series of mutant G alpha 2 subunits expressed from the cloned G alpha 2 promoter. Our results show that wild-type cells expressing G alpha 2 subunits carrying mutations G40V and Q208L in the highly conserved GAGESG (residues 38-43) and GGQRS (residues 206-210) domains, which are expected to reduce the intrinsic GTPase activity, are blocked in multicellular development. Analysis of down-stream effector pathways essential for mediating aggregation indicates that cAMP-mediated activation of guanylyl cyclase and phosphatidylinositol-phospholipase C (PI-PLC) is almost completely inhibited and that there is a substantial reduction of cAMP-mediated activation of adenylyl cyclase. Moreover, neither mutant G alpha 2 subunit can complement g alpha 2 null mutants. Expression of G alpha 2(G43V) and G alpha 2(G207V) have little or no effect on the effector pathways and can partially complement g alpha 2 null cells. Our results suggest a model in which the dominant negative phenotypes resulting from the expression of G alpha 2(G40V) and G alpha 2(Q208L) are due to a constitutive adaptation of the effectors through a G alpha 2-mediated pathway. Analysis of PI-PLC in g alpha 2 null mutants and in cell lines expressing mutant G alpha 2 proteins also strongly suggests that G alpha 2 is the G alpha subunit that directly activates PI-PLC during aggregation. Moreover, overexpression of wild-type G alpha 2 results in the ability to precociously activate guanylyl cyclase by cAMP in vegetative cells, suggesting that G alpha 2 may be rate limiting in the developmental regulation of guanylyl cyclase activation. In agreement with previous results, the activation of adenylyl cyclase, while requiring G alpha 2 function in vivo, does not appear to be directly carried out by the G alpha 2 subunit. Our data are consistent with adenylyl cyclase being directly activated by either another G alpha subunit or by beta gamma subunits released on activation of the G protein containing G alpha 2.

cAMP signal transduction pathways regulating development of Dictyostelium discoideum

Dictyostelium discoideum development is regulated through receptor/G protein signal transduction using cAMP as a primary extracellular signal. Signaling pathways will be discussed as well as the regulation and function of individual cAMP receptors and G alpha subunits. Finally potential downstream targets including protein kinases and nuclear events will be explored.

Identification of Dictyostelium G alpha genes expressed during multicellular development

Guanine nucleotide-binding protein (G protein)-mediated signal transduction constitutes a common mechanism by which cells receive and respond to a diverse set of environmental signals. Many of the signals involved in the developmental life cycle of the slime mold Dictyostelium have been postulated to be transduced by such pathways and, in some cases, these pathways have been demonstrated to be dependent on specific G proteins. Using the polymerase chain reaction, we have identified two additional Dictyostelium G alpha genes, G alpha 4 and G alpha 5, that are developmentally regulated. Transcripts from both of these genes are primarily expressed during the multicellular stages of development, suggesting possible roles in cell differentiation or morphogenesis. The entire G alpha 4 gene was sequenced and found to encode a protein consisting of 345 amino acids. The G alpha 4 subunit is homologous to other previously identified G alpha subunits, including the Dictyostelium G alpha 1 (43% identity) and G alpha 2 (41% identity) subunits. However, the G alpha 4 subunit contains some unusual sequence divergences in residues highly conserved among most eukaryotic G alpha subunits, suggesting that G alpha 4 may be a member of another class of G alpha subunits.

A developmentally regulated, putative serine/threonine protein kinase is essential for development in Dictyostelium

Using PCR technology, we have cloned parts of three developmentally regulated putative serine/threonine kinases from Dictyostelium. All show significant homology to members of the cAMP-dependent protein kinase A/protein kinase C subfamilies. A genomic clone encoding one of these, DdPK3, has been isolated and sequenced. The open reading frame encodes a protein of 648 amino acids with the conserved kinase domain in the C-terminal half. The protein encoded by this gene is unusual in that it contains long homopolymer runs in the N-terminal half of the protein, including a long run of 88 amino acids in which 73 are glutamine residues. To examine the function of DdPK3, a gene disruption was created via homologous recombination. Ddpk3- cells do not aggregate by themselves but will co-aggregate with wild-type cells. However, after aggregation these cells are 'sloughed off' and do not proceed further through development, but are found as a discrete mass alongside the fruiting body formed by the wild-type cells. Analysis of signal transduction pathways indicates that cAMP pulse-induced expression of aggregation stage-specific genes is normal in Ddpk3- cells, as is induction of the prestalk gene Ddras in single cell assays. However, cAMP induction of the late promoters of cAMP receptor cAR1 and of two prespore-specific genes is absent under similar conditions. These cells show normal activation of adenylate cyclase and normal phosphorylation of the G alpha protein G alpha 2 in response to cAMP. The possible role of DdPK3 in Dictyostelium development is discussed.

Disruption of the gene encoding the p34/31 polypeptides affects growth and development of Dictyostelium discoideum

We have used homologous recombination to disrupt the gene which codes for p34 and p31, two polypeptides related to a cAMP-binding protein (CABP1) in Dictyostelium discoideum. By screening a total of 80 independent transformants by Southern blotting, four mutants have been isolated. Two of these mutants were analyzed in detail. Our results indicate that, while a null allele has not been obtained, both mutants express drastically reduced levels of truncated p34 and p31. Phenotypic analysis has demonstrated that both of them grow significantly more slowly than wild-type controls when bacteria are used as a food source. Interestingly, this growth defect is not seen when the cells are cultured axenically. In addition, the mutants possess an altered developmental profile. They complete development approximately 3 h later than wild-type controls. These results indicate that p34 and p31 play roles in both growth and development in this organism.

Gene targeting of the aggregation stage cAMP receptor cAR1 in Dictyostelium

cAR1, a G-protein-linked surface cAMP receptor, plays a central role in the development of Dictyostelium. To investigate its role, we sought to target the cAR1 gene by homogolous recombination. Transformation of these amoebas with appropriately designed vectors results in integration into the cAR1 locus with high frequency. cAR1 "null" mutants, resulting from double crossover events, fail to bind or sense cAMP and arrest in early development. The null mutants can be rescued by constitutive expression of a wild-type cAR1 cDNA. Carboxy-terminal deletion mutants, derived from single crossover events, express a truncated form of cAR1 that binds and senses cAMP. These cells proceed through the developmental program, albeit with a delay. Constitutive expression of a similar truncated form of cAR1 also rescues the null mutant. These observations prove that cAR1-mediated signal transduction controls the development of Dictyostelium and allow structural/functional studies of a G-protein-linked receptor in its natural context.

Identification of a protein kinase multigene family of Dictyostelium discoideum: molecular cloning and expression of a cDNA encoding a developmentally regulated protein kinase

We have identified protein kinase genes of Dictyostelium by using highly conserved amino acid sequence motifs to design the synthesis and amplification of DNA fragments by polymerase chain reactions (PCRs). Cloning and sequencing the PCR products have revealed five different members of the protein kinase multigene family. These five putative kinases showed varying degrees of amino acid sequence similarity (40-70%) to protein kinases in data bases and contained invariant amino acid residues characteristic of protein kinases. DNA from PCR was labeled and used to isolate several lambda gt11 cDNA clones, including one full-length one (Dd kinase-2). The nucleotide sequence of Dd kinase-2 contained a region identical to one of the cloned kinase fragments amplified by PCR, and based on the deduced amino acid sequence Dd kinase-2 encodes a protein of 479 amino acids. A 350-amino acid kinase domain at the C-terminal end shows high homology to the catalytic domains of protein kinase A, protein kinase C, S-6 kinase of Xenopus, and the suppressor of cdc25 of yeast. The N-terminal domain is highly basic and also contains alternating threonine/proline residues. The cDNA hybridized to a single copy gene but to two differentially regulated mRNAs--a 2.0-kilobase mRNA that is expressed in vegetative cells and a 2.2-kilobase mRNA that is expressed during development. The larger mRNA is induced by cAMP by using a cell-surface receptor-mediated signal transduction pathway.

Sensory transduction in eukaryotes. A comparison between Dictyostelium and vertebrate cells

The organization of multicellular organisms depends on cell-cell communication. The signal molecules are often soluble components in the extracellular fluid, but also include odors and light. A large array of surface receptors is involved in the detection of these signals. Signals are then transduced across the plasma membrane so that enzymes at the inner face of the membrane are activated, producing second messengers, which by a complex network of interactions activate target proteins or genes. Vertebrate cells have been used to study hormone and neurotransmitter action, vision, the regulation of cell growth and differentiation. Sensory transduction in lower eukaryotes is predominantly used for other functions, notably cell attraction for mating and food seeking. By comparing sensory transduction in lower and higher eukaryotes general principles may be recognized that are found in all organisms and deviations that are present in specialised systems. This may also help to understand the differences between cell types within one organism and the importance of a particular pathway that may or may not be general. In a practical sense, microorganisms have the advantage of their easy genetic manipulation, which is especially advantageous for the identification of the function of large families of signal transducing components.

Multiple genes for cell surface cAMP receptors in Dictyostelium discoideum

We have cloned and characterized three genes (CAR1, CAR2, CAR3) encoding potential cell surface, cyclic adenosine 3':5' monophosphate (AMP) receptors from Dictyostelium discoideum. The three proteins are predicted to be substantially similar in amino acid sequence throughout most of their transmembrane (TM) and loop domains but are distinctly different in their carboxyl terminal segments. In addition, all three genes possess an intron which interrupts an equivalent codon of TM3. CAR1 is expressed early in development when the cAMP relay system is being established. As development proceeds multiple size forms of CAR1 RNA are detected which apparently result from differences in their 5'-untranslated regions. Late in development levels of CAR1 RNA decrease. In contrast, CAR2 encodes a single sized RNA which is expressed only during postaggregative development. CAR3 expression is approximately 10% of CAR1 during early development, is maximal during tight aggregate formation but declines thereafter. Only one size class of CAR3 mRNA is detected throughout development. Because RNA for each of the three genes is present in postaggregative cells, it was of interest to determine the cell type distribution of each RNA. Gene-specific probes were hybridized to RNAs isolated from cells of Percoll gradient-enriched prespore and prestalk fractions and relative levels of hybridization compared. CAR1 and CAR3 show approximately the same pattern of accumulation; a 3-4 fold enrichment in prestalk cells. CAR2, however, is highly enriched in prestalk cells, more than 10 fold relative to prespore cells.

Homology cloning of protein kinase and phosphoprotein phosphatase sequences of Dictyostelium discoideum

Reversible protein phosphorylation appears to be important at several stages in the signal transduction pathways in Dictyostelium discoideum. To elucidate its role, we have isolated sequences encoding putative protein kinases and phosphoprotein phosphatases by homology cloning using polymerase chain reactions (PCRs). Oligonucleotide primers were synthesized for use as forward and reverse primers with their nucleotide sequences deduced from the amino acid sequences of conserved domains of several protein kinases and phosphoprotein phosphatases. The fragments amplified by PCR were cloned, sequenced, and shown to encode parts of five different protein kinases and two phosphoprotein phosphatases. Several features such as the deduced amino acid sequence homology, location of invariant amino acids, GC content, and the codon usage confirmed that one set of clones encode parts of different protein kinases of Dictyostelium. Two clones derived from phosphoprotein phosphatase primers encode fragments of type 1 and type 2A phosphoprotein phosphatases. Amplified fragments were used to screen a lambda gt11 bank, and several cDNA clones for protein kinases were isolated. Some of these show differential expression during development or in response to exogenous cAMP.

Signal transduction pathways involved in the expression of the uridine diphosphoglucose pyrophosphorylase gene of Dictyostelium discoideum

The uridine diphosphoglucose pyrophosphorylase (UDPGP1) gene of Dictyostelium discoideum is an excellent marker to study the pathways that control the expression of genes during development. We have previously shown that the UDPGP1 gene is regulated by exogenous cAMP acting on cell-surface cAMP receptors. Various steps in the signal transduction pathway between receptor stimulation and the induction of the gene can now be studied. Induction does not require the synthesis of intracellular cAMP, but does require new protein synthesis. By deletion and transformation with altered genes, two cis-acting sequences that are required for UDPGP1 expression have been identified. A GC-rich palindromic sequence located between -410 and -374 is essential for induction of the gene by extracellular cAMP, but not for its basal expression. A sequence element located between -374 and -337 is required for any basal expression of this gene. When the polarity of the palindromic sequence was reversed such that it resembled the H2K enhancer element, the gene could still be induced by exogenous cAMP. Two DNA binding activities were detected in gel mobility shift assays using a fragment containing both of the regulatory sequence elements of UDPGP1 gene. Transformation with a vector that resulted in the synthesis of anti-sense UDPGP1 RNA led to almost total elimination of the enzyme antigen and no detectable enzyme activity. However, these transformants developed normally, indicating that either UDPGP is not required for development or residual synthesis of UDPGP may be sufficient for normal development.

Expression of a cAMP receptor gene of Dictyostelium and evidence for a multigene family

We have previously reported the cloning of cDNAs for a Dictyostelium cell-surface cAMP receptor that is a member of the family of G-protein-linked receptors. Here, we report the organization and the developmental expression of this cAMP receptor gene, designated CAR1. CAR1 is a single copy gene that contains two intervening sequences. CAR1 mRNA levels are low in growing cells, rise to peak expression at 5-10 hr of development when the cAMP signaling system is maximally active, and decrease as development proceeds. At 5 hr the predominant mRNA species is approximately 1.9 kb, by 10 hr the mRNA is heterogeneous with sizes of approximately 1.9-2.1 kb, but during culmination only the 2.1 kb mRNA is detected. The variety of mRNA sizes results from differences in 5'-untranslated regions. Studies using developmental mutants with aberrant cAMP-signaling patterns indicate that pulsatile action of cAMP promotes maximal expression of CAR1 during early development. Low stringency hybridization of CAR1 probes to genomic DNA detects additional, related sequences, suggesting that there are several genes that encode a family of structurally similar receptors. Multiple functions previously attributed to the cAMP receptor instead may be fulfilled by distinct receptor subtypes encoded by specific genes.

Frequency encoding of pulsatile signals of cAMP based on receptor desensitization in Dictyostelium cells

Dictyostelium discoideum amoebae represent a prototype for the study of periodic signaling in intercellular communication. These cells synthesize cAMP in response to cAMP pulses. Cell responsiveness in Dictyostelium can be characterized by the capability to generate a large number of significant responses to cAMP signals in a given amount of time. The existence of a frequency of pulsatile cAMP signals yielding maximum responsiveness is demonstrated by analysis of a realistic model for cAMP synthesis, based on receptor desensitization. The optimal frequency of stimulation closely depends on the kinetics of receptor desensitization and resensitization in target cells. Synthesis of cAMP is determined both in conditions where cells are not excitable and in conditions where they relay suprathreshold pulses of cAMP. Moreover, the effect of the stimulus waveform is investigated, and several measures of cell responsiveness are compared. The results provide an explanation for the effectiveness of cAMP pulses delivered at 5 min intervals, and for the failure of pulses delivered at 2 min intervals, in inducing slime mold development. Besides applying to intercellular communication in Dictyostelium, the present analysis bears on patterns of pulsatile signaling observed for hormones and growth factors. In all these cases, it appears that pulsatile signals can be encoded in terms of their frequency on the basis of desensitization in target cells.

Regulation of the Discoidin I gamma gene in Dictyostelium discoideum: identification of individual promoter elements mediating induction of transcription and repression by cyclic AMP

We dissected the promoter of the developmentally induced and cyclic AMP-repressed discoidin I gamma gene and identified a sequence element essential for developmental induction. Transfer of the element to an inactive heterologous promoter demonstrated that this sequence is sufficient to confer expression in axenically growing cells and to induce gene activity in development after growth on bacteria. A 16-base-pair sequence within this element was shown to be sufficient for induction in the discoidin promoter context and was used to reactivate different truncated promoter constructs. This led to the localization of an element necessary for down regulation of gene expression by extracellular cyclic AMP.

Regulation of the Discoidin I gamma gene in Dictyostelium discoideum: identification of individual promoter elements mediating induction of transcription and repression by cyclic AMP

We dissected the promoter of the developmentally induced and cyclic AMP-repressed discoidin I gamma gene and identified a sequence element essential for developmental induction. Transfer of the element to an inactive heterologous promoter demonstrated that this sequence is sufficient to confer expression in axenically growing cells and to induce gene activity in development after growth on bacteria. A 16-base-pair sequence within this element was shown to be sufficient for induction in the discoidin promoter context and was used to reactivate different truncated promoter constructs. This led to the localization of an element necessary for down regulation of gene expression by extracellular cyclic AMP.

cAMP regulation of the expression of prespore-specific genes, SP96 and Dp87, in disaggregated slug cells of Dictyostelium discoideum

By Northern transfer and in vitro transcription assays, we examined how cAMP controlled the expression of prespore genes, Dp87 and SP96, in disaggregated slug cells of Dictyostelium discoideum. The transcription of these genes was halted soon after disaggregation and the remaining mRNAs were completely lost within 2 h. Addition of cAMP to cells which had lost the mRNAs induced the transcription of these genes, and the mRNAs were re-accumulated after a lag period of 30 min. The cAMP signal was transduced through the cell surface receptor. Protein synthesis was not needed for the induction of the transcription but was required for the accumulation of the mRNAs. We conclude that prespore gene expression is controlled by cAMP in two different ways: direct induction of transcription of the genes, and stabilization of the transcribed mRNAs by a protein(s) synthesized after addition of cAMP.

A pharmacologically distinct cyclic AMP receptor is responsible for the regulation of gp80 expression in Dictyostelium discoideum

The EDTA-resistant cell-cell adhesion expressed at the aggregation stage of Dictyostelium discoideum is mediated by a cell surface glycoprotein of Mr 80,000 (gp80). The expression of gp80 is developmentally regulated by cyclic AMP (cAMP). In vitro nuclear run-on experiments show that transcription of the gp80 gene is initiated soon after the onset of development. The basal level of gp80 transcription is significantly augmented by exogenous cAMP pulses. Interestingly, in analog studies, 2'-deoxy-cAMP, 8-bromo-cAMP, and N6-monobutyryl-cAMP are all capable of inducing a rapid accumulation of gp80 mRNA, suggesting the presence of a unique cAMP receptor that responds equally well to these analogs. To determine whether intracellular cAMP plays a role in the regulation of gp80 expression, caffeine was used to block cAMP-induced receptor-mediated adenylate cyclase activation. Expression of gp80 mRNA was blocked in caffeine-treated cells but could be substantially restored by treatment with exogenous cAMP pulses, suggesting that adenylate cyclase activation is not required. gp80 expression was also examined in the signal transduction mutants synag 7 and frigid A. In both mutants, gp80 was expressed at the basal level. Pulses of cAMP as well as 2'-deoxy-cAMP and N6-monobutyryl-cAMP were capable of restoring the normal level of gp80 expression in synag 7 cells. These results, taken together, indicate bimodal regulation of gp80 expression during development and the involvement of a novel cAMP receptor in the transmembrane signalling pathway that regulates gp80 gene expression.

A pharmacologically distinct cyclic AMP receptor is responsible for the regulation of gp80 expression in Dictyostelium discoideum

The EDTA-resistant cell-cell adhesion expressed at the aggregation stage of Dictyostelium discoideum is mediated by a cell surface glycoprotein of Mr 80,000 (gp80). The expression of gp80 is developmentally regulated by cyclic AMP (cAMP). In vitro nuclear run-on experiments show that transcription of the gp80 gene is initiated soon after the onset of development. The basal level of gp80 transcription is significantly augmented by exogenous cAMP pulses. Interestingly, in analog studies, 2'-deoxy-cAMP, 8-bromo-cAMP, and N6-monobutyryl-cAMP are all capable of inducing a rapid accumulation of gp80 mRNA, suggesting the presence of a unique cAMP receptor that responds equally well to these analogs. To determine whether intracellular cAMP plays a role in the regulation of gp80 expression, caffeine was used to block cAMP-induced receptor-mediated adenylate cyclase activation. Expression of gp80 mRNA was blocked in caffeine-treated cells but could be substantially restored by treatment with exogenous cAMP pulses, suggesting that adenylate cyclase activation is not required. gp80 expression was also examined in the signal transduction mutants synag 7 and frigid A. In both mutants, gp80 was expressed at the basal level. Pulses of cAMP as well as 2'-deoxy-cAMP and N6-monobutyryl-cAMP were capable of restoring the normal level of gp80 expression in synag 7 cells. These results, taken together, indicate bimodal regulation of gp80 expression during development and the involvement of a novel cAMP receptor in the transmembrane signalling pathway that regulates gp80 gene expression.